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1 Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, University Station Box X, Austin, Texas 78713-8924; bob.loucks{at}beg.utexas.edu
2 Veritas, 10300 Town Park Drive, Houston, Texas 77072
3 Center for Lithospheric Studies, University of Texas at Dallas, P.O. Box 830688 (FA31), Richardson, Texas 75083-0688
Robert Loucks obtained his B.A. degree from the State University of New York, Binghamton, in 1967 and his Ph.D. from the University of Texas at Austin in 1976. He is a senior research scientist at the Bureau of Economic Geology, working on carbonate and siliciclastic reservoir characterization research. His major interests include sequence stratigraphy, depositional systems, and diagenesis of both carbonates and siliciclastics.Paul Mescher is a principal consulting geologist for Veritas Exploration Services in Houston, Texas. He has more than 22 years of diverse geological experiences, including prospect generation, field extension and development, and reservoir characterization from many areas of the United States and several countries, including the former Soviet Union, Saudi Arabia, Syria, offshore China, Tunisia, Mexico, and Canada. He is author or coauthor of 13 papers.
George McMechan received a B.A.Sc. degree in geophysical engineering from the University of British Columbia in 1970 and an M.Sc. degree in geophysics from the University of Toronto in 1971. He is a professor at the University of Texas at Dallas. His main research interests are wavefield imaging, three-dimensional seismology, reservoir characterization, and ground-penetrating radar.
The three-dimensional, interwell-scale architecture of a Lower Ordovician Ellenburger coalesced, collapsed-paleocave system was constructed through the integration of ground-penetrating radar (GPR), shallow-core, and outcrop data. The data were collected near Marble Falls in central Texas over an area (
800 x 1000 m [2600 x 3300 ft]) that could cover several oil-well locations (160 ac; 0.65 km2) typical of a region such as west Texas. Integration of core-based facies descriptions with GPR-reflection response identified several paleocave facies that can be recognized and mapped with GPR data alone: (1) continuous reflections image the undisturbed strata, (2) relatively continuous reflections (over tens of meters) characterized by faults and folds image the disturbed strata, and (3) chaotic reflections having little to no perceptible continuity image heterogeneous, cave-related brecciated facies recognized in core that cannot be individually resolved with the GPR data. These latter facies include the highly disturbed strata, coarse-clast chaotic breccia, fine-clast chaotic breccia, and sediment fill.
The three-dimensional architecture of the coalesced, collapsed-paleocave system based on core and GPR data indicates that there are trends of brecciated bodies that are as much as 350 m (1100 ft) wide, greater than 1000 m (3300 ft) long, and tens of meters high. These brecciated bodies are coalesced, collapsed paleocaves. Between the brecciated bodies are areas of disturbed and undisturbed host rock that are jointly as much as 200 m (660 ft) wide.
As a cave system is buried, many structural features form by mechanical compaction. These features include folds, sags, and faults. The folds and sags measure from a few meters to several hundred meters wide. The collapse-related faults are numerous and can have several meters of displacement. Most are normal faults, but reverse faults also occur.
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